I used TextMate's Diff tools all the time. One thing I particularly liked was that you could diff the contents of two tabs, or even one tab with the clipboard, so you don't need to save them as files first.

It's a very handy way to diff a couple of web pages when you can view source, copy and paste into a new tab. View source on the other page and hit the Diff with clipboard command. Boom.

This is a feature I would love to see added. I'm not an expert on diff tools, but I use WinMerge quite a bit - a compromise between its features and ST's design philosophies would be really nice to see.

I think it's important to keep ST as feature-minimal as possible, but if notepad++ has a built-in diff tool I think it's okay to add one ..